Error with Mirror Modifier. What did I wrong?

Hi there, just a short question. It's not about TMT though, but I hope it's ok to post it in this thread anyway

In Blender I've started to create a roof for an old German fairy house. First I've created just one side of the house and wanted to add a mirror modifier. But something went wrong. The mirrored half is overlapping.
Could anyone tell my what i did wrong? Has this possibly to do with the rotation and scale settings which I was adjusting first (-90/180/180 and scale to 100)?